img2grd(1) GMT img2grd(1)
NAME
img2grd - Extract subset of img file in Mercator or Geographic format
SYNOPSIS
img2grd imgfile -Ggrdfile
-Rregion
-Ttype [ -C ] [ -D[minlat/maxlat] ] [ -E ] [ -Iminutes ] [ -M ] [
-Nnavg ] [ -S[scale] ] [ -V[level] ] [ -Wmaxlon ] [ -nflags ]
Note: No space is allowed between the option flag and the associated
arguments.
DESCRIPTION
img2grd reads an img format file, extracts a subset, and writes it to a
grid file. The -M option dictates whether or not the Spherical Mercator
projection of the img file is preserved or if a Geographic grid should
be written by undoing the Mercator projection. If geographic grid is
selected you can also request a resampling onto the exact -R given.
REQUIRED ARGUMENTS
imgfile
A Mercator img format file such as the marine gravity or
seafloor topography fields estimated from satellite altimeter
data by Sandwell and Smith. If the user has set an environment
variable $GMT_DATADIR, then img2grd will try to find imgfile in
$GMT_DATADIR; else it will try to open imgfile directly.
-Ggrdfile
grdfile is the name of the output grid file.
-Rwest/east/south/north[/zmin/zmax][+r][+uunit]
west, east, south, and north specify the region of interest, and
you may specify them in decimal degrees or in
[A+-]dd:mm[:ss.xxx][W|E|S|N] format Append +r if lower left and
upper right map coordinates are given instead of w/e/s/n. The
two shorthands -Rg and -Rd stand for global domain (0/360 and
-180/+180 in longitude respectively, with -90/+90 in latitude).
Alternatively for grid creation, give Rcodelon/lat/nx/ny, where
code is a 2-character combination of L, C, R (for left, center,
or right) and T, M, B for top, middle, or bottom. e.g., BL for
lower left. This indicates which point on a rectangular region
the lon/lat coordinate refers to, and the grid dimensions nx and
ny with grid spacings via -I is used to create the corresponding
region. Alternatively, specify the name of an existing grid
file and the -R settings (and grid spacing, if applicable) are
copied from the grid. Appending +uunit expects projected (Carte-
sian) coordinates compatible with chosen -J and we inversely
project to determine actual rectangular geographic region. For
perspective view (-p), optionally append /zmin/zmax. In case of
perspective view (-p), a z-range (zmin, zmax) can be appended to
indicate the third dimension. This needs to be done only when
using the -Jz option, not when using only the -p option. In the
latter case a perspective view of the plane is plotted, with no
third dimension.
OPTIONAL ARGUMENTS
-C Set the x and y Mercator coordinates relative to projection cen-
ter [Default is relative to lower left corner of grid]. Requires
-M.
-D[minlat/maxlat]
Use the extended latitude range -80.738/+80.738. Alternatively,
append minlat/maxlat as the latitude extent of the input img
file. [Default is -72.006/72.006]. Not usually required since
we can determine the extent from inspection of the file size.
-E Can be used when -M is not set to force the final grid to have
the exact same region as requested with -R. By default, the
final region is a direct projection of the original Mercator
region and will typically extend slightly beyond the requested
latitude range, and furthermore the grid increment in latitude
does not match the longitude increment. However, the extra
resampling introduces small interpolation errors and should only
be used if the output grid must match the requested region and
have x_inc = y_inc. In this case the region set by -R must be
given in multiples of the increment (.e.g, -R0/45/45/72).
-I Indicate minutes as the width of an input img pixel in minutes
of longitude. [Default is 2.0]. Not usually required since we
can determine the pixel size from inspection of the size.
-M Output a Spherical Mercator grid [Default is a geographic
lon/lat grid]. The Spherical Mercator projection of the img file
is preserved, so that the region -R set by the user is modified
slightly; the modified region corresponds to the edges of pixels
[or groups of navg pixels]. The grid file header is set so that
the x and y axis lengths represent distance from the west and
south edges of the image, measured in user default units, with
-Jm1 and the adjusted -R. By setting the default PROJ_ ELLIPSOID
= Sphere, the user can make overlays with the adjusted -R so
that they match. See EXAMPLES below. The adjusted -R is also
written in the grid header remark, so it can be found later. See
-C to set coordinates relative to projection center.
-Nnavg Average the values in the input img pixels into navg by navg
squares, and create one output pixel for each such square. If
used with -T3 it will report an average constraint between 0 and
1. If used with -T2 the output will be average data value or NaN
according to whether average constraint is > 0.5. navg must
evenly divide into the dimensions of the imgfile in pixels.
[Default 1 does no averaging].
-S[scale]
Multiply the img file values by scale before storing in grid
file. [Default is 1.0]. For recent img files: img topo files
are stored in (corrected) meters [-S1]; free-air gravity files
in mGal*10 [-S0.1 to get mGal]; vertical deflection files in
micro-radians*10 [-S0.1 to get micro-radians], vertical gravity
gradient files in Eotvos*50 [-S0.02 to get Eotvos, or -S0.002 to
get mGal/km]). If no scale is given we try to determine the
scale by examining the file name for clues.
-Ttype type handles the encoding of constraint information. type = 0
indicates that no such information is encoded in the img file
(used for pre-1995 versions of the gravity data) and gets all
data. type > 0 indicates that constraint information is encoded
(1995 and later (current) versions of the img files) so that one
may produce a grid file as follows: -T1 gets data values at all
points, -T2 gets data values at constrained points and NaN at
interpolated points; -T3 gets 1 at constrained points and 0 at
interpolated points [Default is 1].
-V[level] (more a|)
Select verbosity level [c]. Particularly recommended here, as it
is helpful to see how the coordinates are adjusted.
-Wmaxlon
Indicate maxlon as the maximum longitude extent of the input img
file. Versions since 1995 have had maxlon = 360.0, while some
earlier files had maxlon = 390.0. [Default is 360.0].
-n[b|c|l|n][+a][+bBC][+c][+tthreshold] (more a|)
Select interpolation mode for grids.
-^ or just -
Print a short message about the syntax of the command, then
exits (NOTE: on Windows just use -).
-+ or just +
Print an extensive usage (help) message, including the explana-
tion of any module-specific option (but not the GMT common
options), then exits.
-? or no arguments
Print a complete usage (help) message, including the explanation
of all options, then exits.
GEOGRAPHIC EXAMPLES
The -M option should be excluded if you need the output grid to be in
geographic coordinates. To extract data in the region -R-40/40/-70/-30
from world_grav.img.7.2 and reproject to yield geographic coordinates,
you can try
img2grd world_grav.img.16.1 -Gmerc_grav.nc -R-40/40/-70/-30 -V
Because the latitude spacing in the img file is equidistant in Mercator
units, the resulting grid will not match the specified -R exactly, and
the latitude spacing will not equal the longitude spacing. If you need
an exact match with your -R and the same spacing in longitude and lati-
tude, use the -E option:
img2grd world_grav.img.16.1 -Gmerc_grav.nc -R-40/40/-70/-30 -E -V
MERCATOR EXAMPLES
Since the img files are in a Mercator projection, you should NOT
extract a geographic grid if your plan is to make a Mercator map. If
you did that you end of projecting and reprojection the grid, losing
short-wavelength detail. Better to use -M and plot the grid using a
linear projection with the same scale as the desired Mercator projec-
tion (see GMT Example 29). To extract data in the region
-R-40/40/-70/-30 from world_grav.img.7.2, run
gmt img2grd -M world_grav.img.7.2 -Gmerc_grav.nc -R-40/40/-70/-30 -V
Note that the -V option tells us that the range was adjusted to
-R-40/40/-70.0004681551/-29.9945810754. For scripting purposes we can
extract this original region string using grdinfo -Ii. Furthermore, we
can also use grdinfo to find that the grid file header shows its region
to be -R0/80/0/67.9666667. This is the range of x,y we will get from a
Spherical Mercator projection using
-R-40/40/-70.0004681551/-29.9945810754 and -Jm1. Thus, to take
ship.lonlatgrav and use it to sample the merc_grav.nc, we can do this:
gmt set PROJ_ELLIPSOID Sphere
gmt mapproject -R-40/40/-70.0004681551/-29.9945810754 -Jm1i ship.lonlatgrav | \
gmt grdtrack -Gmerc_grav.nc | gmt mapproject \
-R-40/40/-70.0004681551/-29.9945810754 -Jm1i -I > ship.lonlatgravsat
It is recommended to use the above method of projecting and unproject-
ing the data in such an application, because then there is only one
interpolation step (in grdtrack). If one first tries to convert the
grid file to lon,lat and then sample it, there are two interpolation
steps (in conversion and in sampling).
To make a lon,lat grid from the above grid we can use
gmt grdproject merc_grav.nc -R-40/40/-70.0004681551/-29.9945810754 -Jm1i -I -D2m -Ggrav.nc
In some cases this will not be easy as the -R in the two coordinate
systems may not align well. When this happens, we can also use (in
fact, it may be always better to use)
gmt grd2xyz merc_grav.nc | gmt mapproject \
-R-40/40/-70.0004681551/-29.994581075 -Jm1i -I | \
gmt surface -R-40/40/-70/70 -I2m -Ggrav.nc
To make a Mercator map of the above region, suppose our gmt.conf value
for PROJ_LENGTH_UNIT is inch. Then since the above merc_grav.nc file is
projected with -Jm1i it is 80 inches wide. We can make a map 8 inches
wide by using -Jx0.1i on any map programs applied to this grid (e.g.,
grdcontour, grdimage, grdview), and then for overlays which work in
lon,lat (e.g., psxy, pscoast) we can use the above adjusted -R and
-Jm0.1 to get the two systems to match up.
However, we can be smarter than this. Realizing that the input img file
had pixels 2.0 minutes wide (or checking the nx and ny with grdinfo
merc_grav.nc) we realize that merc_grav.nc used the full resolution of
the img file and it has 2400 by 2039 pixels, and at 8 inches wide this
is 300 pixels per inch. We decide we do not need that many and we will
be satisfied with 100 pixels per inch, so we want to average the data
into 3 by 3 squares. (If we want a contour plot we will probably choose
to average the data much more (e.g., 6 by 6) to get smooth contours.)
Since 2039 isnat divisible by 3 we will get a different adjusted -R
this time:
gmt img2grd -M world_grav.img.7.2 -Gmerc_grav_2.nc -R-40/40/-70/-30 -N3 -V
This time we find the adjusted region is
-R-40/40/-70.023256525/-29.9368261101 and the output is 800 by 601 pix-
els, a better size for us. Now we can create an artificial illumination
file for this using grdgradient:
gmt grdgradient merc_grav_2.nc -Gillum.nc -A0/270 -Ne0.6
and if we also have a CPT called agrav.cpta we can create a color
shaded relief map like this:
gmt grdimage merc_grav_2.nc -Iillum.nc -Cgrav.cpt -Jx0.1i -K > map.ps
gmt psbasemap -R-40/40/-70.023256525/-29.9368261101 -Jm0.1i -Ba10 -O >> map.ps
Suppose you want to obtain only the constrained data values from an img
file, in lat/lon coordinates. Then run img2grd with the -T2 option, use
grd2xyz to dump the values, pipe through grep -v NaN to eliminate NaNs,
and pipe through mapproject with the inverse projection as above.
SEE ALSO
gmt(1)
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 img2grd(1)
gmt5 5.4.2 - Generated Thu Jun 29 13:44:34 CDT 2017